Adds Track-Only App Support (Addresses #119 and Sets Groundwork for #44) (#123)

- All Sources now have a "Track-Only" option that will prevent Obtainium from looking for APKs (though the App must still have a release of some kind so that a version string can be grabbed).
    - These Apps cannot be installed through Obtainium, but update notifications will still be sent.
    - The user needs to manually mark them as updated when appropriate.
    - This addresses issue #119.
    - It also partially addresses #44 by allowing some sources to be configured as "Track-Only"-only. The first such source (APKMirror) will be added later.
- Includes various UI changes to accommodate the above change.
- Also makes App loading a bit more responsive (sending Obtainium to the background then returning will now cause App re-load to pick up changes in App versioning that may have been made in the meantime, for instance through update checking).

lib/app_sources/gitlab.dart

@@ -33,7 +33,7 @@ class GitLab extends AppSource {
       var entry = parsedHtml.querySelector('entry');
       var entryContent =
           parse(parseFragment(entry?.querySelector('content')!.innerHtml).text);
-      var apkUrlList = [
+      var apkUrls = [
         ...getLinksFromParsedHTML(
             entryContent,
             RegExp(
@@ -48,9 +48,6 @@ class GitLab extends AppSource {
             .where((element) => Uri.parse(element).host != '')
             .toList()
       ];
-      if (apkUrlList.isEmpty) {
-        throw NoAPKError();
-      }
 
       var entryId = entry?.querySelector('id')?.innerHtml;
       var version =
@@ -58,7 +55,7 @@ class GitLab extends AppSource {
       if (version == null) {
         throw NoVersionError();
       }
-      return APKDetails(version, apkUrlList);
+      return APKDetails(version, apkUrls);
     } else {
       throw NoReleasesError();
     }